1. Field of the Invention
This invention relates to a process for the production of very fine-particle alkali metal aluminium silicates, more particularly zeolites of the A type, of the P type and of the faujasite type (X and Y).
2. Statement of Related Art
Alkali metal aluminium silicates of the type in question are used, for example, as builders for laundry detergents, dishwashing detergents and cleaning formulations, as molecular sieves, catalysts and absorbents and as additives in ceramic frits and glazes.
Two important criteria for the use of zeolites in laundry detergents, dishwashing detergents and cleaning compositions and as additives in ceramic frits and glazes are the mean particle size (expressed as the d.sub.50 value) and the width of the particle size distribution spectrum. The d.sub.50 value indicates that 50% by weight of the measured sample consists of particles which are smaller than the indicated value. There are various methods for measuring the d.sub.50 value. The measured values obtained by the various methods cannot readily be compared with one another. In the context of the present invention, the d.sub.50 value is measured by light diffraction and light scattering methods. This procedure is described, for example, in H. Wachernig, Sprechsaal, Vol. 121, No. 3, 1988, pages 213 to 215.
Another important criterion for the use of zeolites in laundry detergents, dishwashing detergents and cleaning compositions is their cation exchange capacity, more particularly their exchange capacity with respect to calcium ions. The corresponding values were determined by adding the corresponding zeolite to a calcium chloride solution and, after filtration, subjecting the remaining calcium ions present in the solution to complexometric titration.
There are various known processes for the production of very fine-particle zeolites of the zeolite A type.
DE-A-29 51 192 describes a process for the production of zeolite A with a small and uniform particle size. The particle size is controlled on the basis of the following parameters:
1) the stoichiometric SiO.sub.2 /Al.sub.2 O.sub.3 ratio, PA1 2) the stoichiometric H.sub.2 O/Na.sub.2 O ratio, PA1 3) the crystallization time of the amorphous alumosilicate gel at a given temperature, PA1 4) the stoichiometric Na.sub.2 O/Al.sub.2 O.sub.3 ratio and PA1 5) the concentration of Al.sub.2 O.sub.3. PA1 Me is sodium and/or potassium, PA1 x is a number of 0.8 to 1.3, PA1 y is a number of 1.3 to 10 and PA1 z is a number of 0 to 6, PA1 (a) the reaction components are mixed together with intensive stirring over a period of less than 10 minutes at a temperature in the range from 20.degree. to 70.degree. C., at least one of the two reaction components being used in less than the stoichiometrically necessary quantity or both reaction components being used in the stoichiometrically necessary quantity, PA1 (b) the reaction mixture is stirred for 5 to 40 minutes at the temperature established in step (a), PA1 (c) after the ageing step (b), the reaction mixture is heated with stirring for 5 to 30 minutes to a temperature of more than 70.degree. C. to the boiling point of the reaction mixture, PA1 (d) the amorphous alkali metal aluminium silicate gel obtained is allowed to crystallize for at least 10 minutes at the temperature established in step (c), PA1 (e) 2 to 50 mole-% of at least one of the two reaction components, based on the quantity of the reaction components already used in step (a), being added to the reaction mixture during the crystallization step (d) either to make up the balance to the stoichiometrically necessary quantity or as a stoichiometric excess. PA1 in the range from 1.35 to 2.35 for zeolites of the A type, PA1 in the range from 1.85 to 3 for zeolites of the P type and PA1 in the range from 2.5 to 10 for zeolites of the faujasite type. PA1 smaller than 4 .mu.m for zeolites of the A type, PA1 smaller than 2 .mu.m for zeolites of the P type and PA1 smaller than 6 .mu.m for zeolites of the faujasite type.
The particle size of the zeolite formed after crystallization is reduced by increasing the solids concentration on the one hand and by increasing the Na.sub.2 O/Al.sub.2 O.sub.3 ratio in the reaction mixture on the other hand, i.e. in the solutions which, after they have been combined, form an amorphous alumosilicate gel. This process has its natural limits: if, for example, the solids concentration is excessively increased, the particle size spectrum expands. If the Na.sub.2 O/Al.sub.2 O.sub.3 ratio is too high, the time between complete reaction to zeolite 4A and the following conversion into hydroxysodalite is very short so that there is not enough time for subsequent processing of the zeolite suspension.
It is known from DE-A-27 04 310 that, after the reaction components involved in the production of alkali metal aluminium silicates have been mixed with intensive stirring, the intensive stirring can be continued at least until the viscosity maximum is exceeded or, optionally, the viscosity maximum is not fully reached, after which the suspensions are recycled at least once through a size-reducing unit and are then optionally kept at elevated temperature pending crystallization.
It is known from DE-A-27 34 296 that, in the production of fine-particle sodium aluminium silicates, the suspension obtained after the aqueous sodium aluminate solution has been rapidly mixed with the aqueous sodium silicate solution can be briefly stirred at the same temperature before the crystallization step is carried out at elevated temperature.
DE-A-30 11 834 describes a process for the production of very fine-particle zeolitic sodium aluminium silicates, in which steam is introduced during discontinuous crystallization and, at the same time, the suspension is stirred with multiple-stage high-shear stirrers.
DE-A-29 41 636 also relates to a process for the continuous production of very fine-particle zeolitic sodium aluminium silicates, in which suspensions of the X-ray amorphous sodium aluminium silicate flow continuously through an at least 20-stage crystallization reactor acting and/or constructed in stages, the suspensions being kept at a temperature of 80.degree. to 100.degree. C. in the reactor until the degree of crystallization of the zeolitic sodium aluminium silicate has reached at least 80% of the theoretically obtainable crystallinity.
DE-A-26 51 419, 26 51 420, 26 51 436, 26 51 437, 26 51 445 and 26 51 485 describe processes for the production of fine-particle crystalline zeolite powders of the zeolite A type, in which precipitation of the sodium aluminium silicate gel initially formed is delayed by mixing the reaction components in steps over a relatively long period. The gel obtained is then crystallized at a temperature which is higher than the precipitation temperature. These processes give comparatively low volume/time yields.
DE-A-30 07 044, 30 07 080, 30 07 087 and 30 07 123 describe similar processes in which the reaction components are also mixed together in several steps with forming or ageing intervals in between. In these cases, too, crystallization takes place subsequently at a higher temperature. The crystalline zeolites obtained have mean particle sizes of up to 9 .mu.m (cf. Comparison Example 9 below).
Processes for the production of particularly fine-particle products are also known for zeolites of the faujasite type.
U.S. Pat. No. 3,516,786 describes a process in which particularly small particles are obtained by additions of 0.1 to 20% of an organic solvent, such as methanol or dimethyl sulfoxide, to an alumosilicate gel before the crystallization step. On account of the toxicity of the solvents mentioned, special precautions have to be taken in the working-up of the process liquors.
There are several structurally different variants of zeolite P, differing from one another above all in the symmetry of the crystal lattice. P zeolites of the P.sub.c type (also described as P1, B1, cubic form) and of the P.sub.t type (also described as P2, tetragonal form) and mixed crystals thereof or mixtures of both forms are preferably used for applications in laundry detergents, dishwashing detergents and cleaning compositions. The Si:Al ratio is between 1.3:1 and 0.9:1 for a calcium binding capacity of 130 to 165 mg CaO per g of water-free alumosilicate.